Muscular dystrophy is a group of inherited disorders characterized by progressive muscle weakness and loss of muscle tissue. Muscular dystrophies includes many inherited disorders, including Becker's muscular dystrophy and Duchenne's muscular dystrophy, which are both caused by mutations in the dystrophin gene. Both of the disorders have similar symptoms, although Becker's muscular dystrophy is a slower progressing form of the disease. Duchenne's muscular dystrophy is a rapidly progressive form of muscular dystrophy.
Both disorders are characterized by progressive muscle weakness of the legs and pelvis which is associated with a loss of muscle mass (wasting). Muscle weakness also occurs in the arms, neck, and other areas, but not as severely as in the lower half of the body. Calf muscles initially enlarge (an attempt by the body to compensate for loss of muscle strength), the enlarged muscle tissue is eventually replaced by fat and connective tissue (pseudohypertrophy). Muscle contractions occur in the legs and heels, causing inability to use the muscles because of shortening of muscle fibers and fibrosis of connective tissue. Bones develop abnormally, causing skeletal deformities of the chest and other areas. Cardiomyopathy occurs in almost all cases. Mental retardation may accompany the disorder but it is not inevitable and does not worsen as the disorder progresses. The cause of this impairment is unknown. Becker's muscular dystrophy occurs in approximately 3 out of 100,000 people. Symptoms usually appear in men between the ages of 7 and 26. Women rarely develop symptoms. There is no known cure for Becker's muscular dystrophy. Treatment is aimed at control of symptoms to maximize the quality of life. Activity is encouraged. Inactivity (such as bed rest) can worsen the muscle disease. Physical therapy may be helpful to maintain muscle strength. Orthopedic appliances such as braces and wheelchairs may improve mobility and self-care. Becker's muscular dystrophy results in slowly progressive disability. A normal life span is possible; however, death usually occurs after age 40.
Duchenne's muscular dystrophy occurs in approximately 2 out of 10,000 people. Symptoms usually appear in males 1 to 6 years old. Females are carriers of the gene for this disorder but rarely develop symptoms. There is no known cure for Duchenne's muscular dystrophy. Treatment is aimed at control of symptoms to maximize the quality of life. Activity is encouraged. Inactivity (such as bed rest) can worsen the muscle disease. Physical therapy may be helpful to maintain muscle strength and function. Orthopedic appliances such as braces and wheelchairs may improve mobility and the ability for self-care. Duchenne's muscular dystrophy results in rapidly progressive disability. By age 10, braces may be required for walking, and by age 12, most patients are confined to a wheelchair. Bones develop abnormally, causing skeletal deformities of the chest and other areas. Muscular weakness and skeletal deformities contribute to frequent breathing disorders. Cardiomyopathy occurs in almost all cases. Intellectual impairment is common but is not inevitable and does not worsen as the disorder progresses. Death usually occurs by age 15, typically from respiratory (lung) disorders.
Although there are no available treatments for muscular dystrophy, the usefulness of gene replacement as therapy for the disease has been established in transgenic mouse models. Unfortunately, progress toward therapy for human patients has been limited by lack of a suitable technique for delivery of such vectors to large masses of muscle cells. What is needed in the art is a vector that can carry most of the dystrophin coding sequence, that can be cheaply produced in large quantities, that can be delivered to a large mass of muscle cells, and that provides stable expression of dystrophin after delivery.